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1
To make the vinaigrette: Whisk the lemon juice, vinegar, and mustard together in a bowl.
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2
Add the oils in a slow stream, whisking vigorously, to form a light emulsion.
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3
Season to taste with salt and pepper, and set aside.
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4
Bring a pot of water to a rolling boil.
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5
Cut the stem core out of the top of the tomatoes and cut an X in the bottom.
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6
Plunge the tomatoes into the boiling water for 15-20 seconds, then rinse under cold water.
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7
The peel can now be removed easily.
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8
Slice the tomatoes in half crosswise and remove the seeds and pulp.
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9
Put the tomatoes, garlic, thyme, salt and pepper to taste, and the vinaigrette into a food processor or blender and process until well blended.
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10
Thin with a little vinegar if necessary.
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11
Allow the tuna fillets to stand at room temperature for 15 minutes.
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12
Season all sides with salt and pepper.
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13
Heat the 3 tablespoons of olive oil in a large, heavy skillet until almost smoking.
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14
Place 4 tuna blocks at a time in the pan and cook over high heat for about 20 seconds per side, until all sides are lightly browned.
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15
Repeat until all of the tuna is cooked, adding more olive oil if necessary.
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16
Toss the arugula or spring mix with enough vinaigrette to coat and arrange in the center of the plate.
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17
Place tuna around the salad and drizzle with more of the vinaigrette.
